A number of refractive surgery operations involve the removal of small amounts of corneal tissue to effect a refractive change. In earlier developed laser operations such as photorefractive keratectomy (PRK), the epithelial layer is manually removed and then Bowman's membrane and part of the stroma are ablated by laser energy. This operation can lead to unwanted side effects, such as corneal haze, and is not suitable for higher refractive errors.
A more recently developed refractive operation called laser in situ keratomiluesis (LASIK) maintains the integrity of the epithelial layer and Bowman's membrane. A flap of tissue is displaced from the anterior cornea and a minute amount of tissue in the shape of a lens is removed from the stromal bed or from the underside of the lenticule, by way of laser ablation. The corneal lenticule or “flap” is then replaced to its original position creating an optical correction.
The LASIK refractive surgery procedure produces fewer side effects and quicker healing relative to PRK, as the epithelium and Bowman's membrane are left intact. Because the epithelium is generally not touched in LASIK, there is also less pain for the patient and the healing process is enhanced with LASIK, presumably because the healing region is protected: there is moreover only one type of tissue involved in the healing process instead of two. It is thought that about 80% of laser eye surgery is now LASIK rather than PRK.
Notwithstanding these advantages, a known problem with LASIK is that the corneal surface formed by the microkeratome cut is relatively rough and this reduces the patient's vision for watching the fixation light, important for maintaining the pupil correctly aligned, and furthermore reduces the vision of the eye tracker device used to guide the laser delivery system. It is thought that these two issues contribute to the known outcome that the statistical success rate of LASIK is similar to PRK despite the enhanced healing process.
It is therefore an object of the invention, in a preferred application, to address the aforementioned problems with LASIK in an effort to more consistently achieve the outcome expected of LASIK relative to PRK. It is a more general object to provide an improved ultraviolet laser ablation procedure.